From the electrical shunting of a loudspeaker to active impedance control

Variable acoustic properties can be obtained on an electroacoustic transducer’s voicing face by very basic control strategies, among which are the shunting of the loudspeaker (shortcut, variable electric load, or negative resistance disposals). It is proven that enhanced controllability and better performances can be obtained by way of hybrid feedback control, consisting in a double feedback loop, one on acoustic pressure at the diaphragm and the other on its velocity, leading to a global acoustic impedance control. The present work will describe the theory of the hybrid feedback control, by way of block diagrams aiming at visualizing the control principle, starting from the shunted loudspeaker. Simulations of performances obtained on Matlab/Simulink will then be presented, and compared to experimental results obtained on analog prototypes mounted at the end of a dedicated impedance tube. A discussion on stability will then follow, leading to concluding remarks on the disposal behavior and possible means of enhancements.